In recent years, worldwide restrictions on exhaust gas are becoming tighter from the viewpoint of environmental protection. Exhaust gas purification catalysts are being employed in internal combustion engines as one measure. In order to efficiently remove the hydrocarbons (hereunder abbreviated as “HC”), CO and nitrogen oxides (hereunder abbreviated as “NOx”) in exhaust gas, exhaust gas purification catalysts employ precious metals such as Pt, Pd and Rh as catalyst components.
Vehicles using such exhaust gas purification catalysts, such as gasoline engine vehicles and diesel engine vehicles, employ various types of systems designed to increase both catalytic activity and fuel efficiency. For example, in order to increase fuel efficiency, combustion is conducted under lean air/fuel ratio (A/F) conditions (oxygen excess) during steady operation, and in order to increase catalytic activity, combustion is temporarily conducted under stoich (stoichiometric air/fuel ratio, A/F=14.7) to rich (fuel excess) conditions.
This is because conventionally known catalysts including precious metals such as Pt, Pd and Rh have low NOx purification performance under oxidizing conditions, and require a reducing atmosphere by addition of HC or CO to increase purification performance. Because of the effects on catalytic activity it is not possible to increase the air/fuel ratio (A/F) during steady operation, and there has been a limit to increased fuel efficiency with catalysts such as precious metals.
Thus, with conventionally known catalysts such as precious metals, it is necessary to have a fuel that temporarily creates a reducing atmosphere for the purification catalyst, and to lower the air/fuel ratio (A/F) in the engine, and in order to increase the fuel efficiency of internal combustion engines such as vehicle engines, there has been a demand for new purification catalysts that can exhibit NOx purification performance under a stoich atmosphere, for example.
Various modifications have been attempted with the goal of increasing the performance of NOx purification catalysts.
PTL 1 describes an exhaust gas purification catalyst comprising a support made of a metal oxide, and metallic particles supported on the support, wherein the metallic particles are composed of a solid solution of at least one type of first metal selected from the group consisting of Pt, Ag, Mn, Fe and Au, and at least one type of second metal selected from the group consisting of Pd, Rh, Ir and Ru, the mean primary particle size of the metallic particles being 1.5 nm or less, and the standard deviation of the metal compositional ratio for each primary particle of the metallic particles being 10% or less.
However, while PTL 1 mentions Pt and Pd metallic particles as the solid solution, it does not concretely disclose, for example, preventing oxidation of rhodium or lowering the NOx purification temperature.